SMSGT Mac, an oft commenting Defense Tech reader, has a detailed analysis on the recent Army "extreme dust test" of the M4, XM8, Mk16 and HK 416.
It's waaay more detailed than I could get into, and my hat's off to him for his input posted on his blog: Elements of Power. I would strongly suggest taking some time to parse his logic. He's much more cautious in his approach to the test results and seems to say they're not yet a "call to action" for a replacement. I'd agree to some extent: the MRAP debate taught us to carefully consider major equipment overhauls...especially when the justification is more emotional that tactical.
But I would say two things: One, the fact that soldiers aren't "complaining" about the M4 means something, but not much. What do that have to compare the M4 to? It's not like Soldiers get a chance to use an XM8 in combat. And the fact that SOCOM is going to the Mk16 and Mk17 says something, right? The highest trained operators on the planet don't want to have anything to do with the M4...and reliability in austere environments plays a big part of that decision.
Also, the argument about stoppages is not new. If there's a better operating system out there then why not use it? I mean, Colt even has a gas piston reciever they can start building M4s with tomorrow? Is there ANY evidence that system would be in any way WORSE?
Anyway, take some time to look at SMSGT Mac's dissection. And be sure to come back here and give me your $.02 on what he says.